How to Design a Pneumatic Cylinder for Fast Race Car Lifting?

[dmp1511]

Hello

I want to design a pneumatic cylinder for race car lifting. The cylinder should work in such a way that it can lift the car 4 inch from the ground in 0.5 sec. And the max pressure is 250 psi. Can anybody please help me out of this. What are the factors that I have to take into consideration while designing?

 

[DTM]

Pneumatic cylinders that do such a job are right off the shelf. Not sure why you'd want to design one from scratch when companies have put lot of effort into optimizing the design and manufacturing of pneumatic cylinders. Obviously even if you buy one off the shelf, you'll still need to design a proper fixtures for attaching the cylinder to the ground and the car. But if you really want to design the cylinder from scratch, here are some thoughts. Determine the weight you have to lift. Are you lifting 1/2 of the car's weight? Or 1/4? Keep in mind the CG of the car. To travel the 4" in about 1/2 a sec. you'll need a static load capability of maybe 2 or 3 times the load you are lifting. you'll also need fairly large tubing/valves/ports so the flow rate won't slow you down much. Let's just assume the force it take to lift the portion of the car you want to lift is 1000 lbs. I'd design the cylinder to produce 3000 lbs of force. If you have no problem getting 250psi air, than the piston area you need is A = F/P = 3000 lbs/250 psi = 12 in². That equates to a 3.9" piston diameter.
Also keep in mind the rod diameter. If you're pressurizing the side of the cylinder w/ the rod, you have to subtract the rod area from you piston area to get your force. But I would think you could design it so you pressurize the non-rod side.
Ideally you don't want to put the rod in bending. If you can insure the car will not move laterally by other means (blocking the wheels on the ground) that would make it easier to design the rod. If that is not practical, you'd want to design the rod to handle the bending load.
Good luck and feel free to post sketches as you come through the design if you want a review.

 

[dmp1511]

Thank you very much for reply...I have to design 4 cylinders equally loaded and the weight of the car is known which is 1500 pounds...can I take the Factor of Safety as 1.5 time the total weight and than do further design?

 

[DTM]

Since you're using 4 cylinders, that's 4x the flow area, I can see a smaller than 3x factor working. Most of the pneumatic cylinders I've used, I typically use 1.5-2x the static loading for the design load. And I don't think they would go 4" in .5 seconds. However, I'm also used to using only 100 psi. Calculating the speed is a really difficult thing since it's much more than just F=ma. It involves the flow which is really up to your air supply hoses, cylinder ports, etc. My gut says a 2x factor will get you there, so if you design each cylinder to provide an 800 lbs load, that may get you the speed you want.